Magnetic tape recorder/players of the type applicable to the present invention are typically used to back-up data stored on nonremovable disks in computers. Nonremovable disks are very convenient, but stored data may be lost due to operator error or mechanical or electrical failure. Thus, it is safest to periodically back-up the data stored on these nonremovable disks by recording it on another nonvolatile memory medium.
The magnetic tape recorder/player provides an answer to the problem of backing up nonremovable disks by storing data on a magnetic tape within a cartridge. A single tape cartridge will usually have sufficient capacity to back up all the data on a single nonremovable disk, but there is an ever increasing need for cartridges having larger and larger data capacities. Such increasing data densities rely on smaller data tracks on the magnetic tape which, in turn, require more precise head positioning for reliable data retrieval.
High-capacity magnetic tape recorder/players incorporate multiple read and/or write/erase heads, typically mounted on a single head block. Ideally the heads are in perfect alignment with the centerline of tape movement so there are no errors introduced as the tape moves relative to the heads. Usually, however, errors are introduced in manufacturing the tape cartridge or the recorder/player. One measurement of this error is called tape skew, which takes into account the angle that the tape makes with respect to its own cartridge (slope), the angle that the cartridge makes with respect to the heads, and azimuth, the angle the gap of a head makes with respect to a data track.
At any point on the tape the skew tends to be in the same direction, so if the tape is always written in the same direction some skew problems would be minimized. In practice however, tapes are written and played in both directions. This can lead to several technical problems. The read head may actually leave the writer path, resulting in a serious problem which cannot be corrected. Another symptom of tape skew manifests itself as tracks being written offset relative to one another. For example, if skew is sufficiently pronounced, the data track closest to the center of the tape written in one direction may collide with the data track closest to the center of the tape written in the opposite direction. This problem is not solved by increasing the gap between the two center tracks because increasing the gap could push the outermost tracks off the edge of the tape and limit capacity. Clearly, if magnetic tape cartridges are to continue their increase in capacity an accurate and reliable means to correct for skew errors is required.
Various techniques have been proposed to correct for at least some aspects of tape misalignment. For example, according to Cahoon et al., U.S. Pat. No. 4,563,713, the read head is used to scan the beginning of the tape and locate the lower and upper edges of a reference track. From these edges, the centerline of the track is determined and used for accurate positioning of the head block onto other tracks. This technique is limited by the fact that a track, which may itself be misaligned, is used as a reference position.
The U.S. Pat. No. 4,866,548 to Rudi improves upon the Cahoon method by using the edge of the tape, as a reference position. After locating the edge, a stripe is written on the tape and the outer edges of the stripe are used to calculate a midpoint value. The distance between write position and midpoint of read positions is used to calculate an error value. This error value is then compared to the value used to position the head block during writing and the difference, if any, is used to more accurately position the head block onto the other tracks.
One disadvantage of the Rudi method is that both edges of a reference stripe must be located. In practice, if the lateral dimensions of the read and write/erase heads are known, a more general method is possible which involves the location of only one edge of a reference stripe or multiple stripes, which may then be averaged with other error calculations, if necessary, to determine an overall correction factor.